Polysaccharid

Polysaccharid, or polysaccharide, is a high-molecular-weight carbohydrate composed of long chains of monosaccharide units linked by glycosidic bonds. They are among the most abundant biological macromolecules and perform diverse roles, principally energy storage and structural support.

They are classified as homopolysaccharides, containing a single type of monosaccharide (for example starch, glycogen, cellulose), or heteropolysaccharides, containing two or more monosaccharides (such as pectins, hemicelluloses, and glycosaminoglycans). Starch and glycogen store glucose in plants and animals, respectively, with starch comprising amylose (mostly linear) and amylopectin (branched). Cellulose consists of β-D-glucose units linked by β-1,4 bonds to form strong, linear fibers; it provides structural support in plants and is not digestible by humans. Chitin, based on N-acetylglucosamine, forms the exoskeletons of arthropods and fungal cell walls.

Linkages determine properties: α-glycosidic bonds (as in starch and glycogen) create compact, energy-dense, often branched polymers that are digestible by many organisms; β-linkages (as in cellulose) yield rigid, unbranched fibers that resist human digestion.

Beyond energy and structure, polysaccharides participate in cell signaling, protection, and interaction with other biomolecules. They are widely used commercially as thickeners or gelling agents (pectins, agar, guar gum) and include medically important sulfated polysaccharides such as heparin and hyaluronic acid.